Harmonic trap filter using coupled resonators

ABSTRACT

A harmonic trap filter suppresses at least one harmonic signal produced by an amplifier and includes an input terminal and a ground terminal. The harmonic trap filter further includes a plurality of resonators electrically coupled one to another between the input terminal and the ground terminal in a spatial order defined by relative phase shift of alternating voltage bias signals respectively applied thereto. The resonators are tuned to resonate at a frequency at which a phase delay is imparted to the at least one harmonic signal by the resonators to effect cancelation of the at least one harmonic signal at the input terminal.

BACKGROUND

High power amplifiers often generate unwanted harmonic content due tononlinearities inherent in the circuit components from which theamplifiers are built. In radar, for example, amplifier harmonics cancause elevated sidelobes as well as a mischaracterization of the actualrange ambiguity function. Obviously, to avoid such system anomalies, theharmonic content must be filtered out. Such additional filtering isusually realized in series with the high power amplifier output, thuslowering the desired output level by filter insertion loss. Thisadditional filtering is thus costly in terms of power loss and dollars.

SUMMARY

A harmonic trap filter suppresses a harmonic signal produced by anamplifier and includes an input terminal and a ground terminal. Thefilter further includes a plurality of resonators electrically coupledone to another between the input terminal and the ground terminal in aspatial order defined by relative phase shift of alternating voltagebias signals respectively applied thereto. The resonators are tuned toresonate at a frequency at which a phase delay is imparted to theharmonic signal by the resonators to effect cancelation of the harmonicsignal at the input terminal.

A radio-frequency (RF) transmitter comprises an amplifier and a harmonictrap filter to suppress at least one harmonic signal produced by theamplifier. The harmonic trap filter includes an input terminal, a groundterminal and a plurality of resonators electrically coupled one toanother between the input terminal and the ground terminal in a spatialorder defined by relative phase shift of alternating voltage biassignals respectively applied thereto. The resonators are tuned toresonate at at least one frequency at which a phase delay is imparted tothe at least one harmonic signal by the resonators to effect cancelationof the at least one harmonic signal at the input terminal.

A shunt harmonic trap filter comprises an input terminal, a firstmagnet-free isolator circuit and a second magnet-free isolator circuitelectrically coupled in series with the first magnet-free isolatorcircuit to define a non-reciprocal circuit path that begins and ends atthe input terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a transmitter by which theprinciples of the present disclosure may be embodied.

FIG. 2 is an electrical schematic diagram of an example harmonic trapfilter by which the principles described herein can be embodied.

FIG. 3 is an electrical schematic diagram of an example variablecapacitance circuit that may be used to embody the principles of thisdisclosure.

FIG. 4 is an electrical schematic diagram of a harmonic trap filter at ahigher level of abstraction than that illustrated in FIG. 2.

FIG. 5 is a graph of simulated transmission coefficient S(2,1) of oneembodiment of the principles of this disclosure.

FIG. 6 is an electrical schematic diagram of another harmonic trap bywhich principles of the present disclosure are embodied.

FIG. 7 is a graph of simulated transmission coefficient S(2,1) of theembodiment illustrated in FIG. 6.

DETAILED DESCRIPTION

The present concept is best described through certain embodimentsthereof, which are described in detail herein with reference to theaccompanying drawings, wherein like reference numerals refer to likefeatures throughout. It is to be understood that the term invention,when used herein, is intended to connote the concept underlying theembodiments described below and not merely the embodiments themselves.It is to be understood further that the general concept is not limitedto the illustrative embodiments described below and the followingdescriptions should be read in such light.

Additionally, the word exemplary is used herein to mean, “serving as anexample, instance or illustration.” Any embodiment of construction,process, design, technique, etc., designated herein as exemplary is notnecessarily to be construed as preferred or advantageous over other suchembodiments. Particular quality or fitness of the examples indicatedherein as exemplary is neither intended nor should be inferred.

FIG. 1 is a schematic block diagram of a transmitter 100 by which theprinciples of the present disclosure may be embodied. Such a transmitter100 may be found in numerous applications including radar andtelecommunications. Transmitter 100 may include transmitter circuitry110 by which a waveform 112 is generated. Waveform 112 is provided toamplifier 150, which generates transmitter waveforms 152, Transmitterwaveforms 152 can include not only the desired amplified signal, butalso undesired artifacts, e.g., harmonics of waveform 112. Harmonic trapfilter 130 may be constructed or otherwise configured to removeharmonics at its input node 154 by way of the principles describedherein. The filtered waveform, which can be an amplified version ofwaveform 112, may be provided to an antenna 140 or other mechanism forconveying the signal over a medium.

Transmitter 100 may include control circuitry 120 by which operations oftransmitter 100 are coordinated. As illustrated in the FIG. 1, controlcircuitry 120 may provide signals to harmonic trap filter 130, such asAC bias signals 122 and DC bias signals 124. Each of AC bias signals 122and DC bias signals 124 may be dynamically altered for filtering signalsthat have non-stationary frequency characteristics, e.g., signalsundergoing linear frequency modulation (LFM) also referred to as chirpmodulation.

FIG. 2 is an electrical schematic diagram of an example harmonic trapfilter 130 by which the principles described herein can be embodied. Itis to be understood that the circuit illustrated in FIG. 2 is but onepossible topology by which the harmonic trap filter functionalitydescribed herein can be realized. The harmonic trap filter function maybe achieved using non-reciprocal features, where, as used herein,non-reciprocity refers to the case where the response of a system isdifferent when the source and receiver are interchanged. Coupledresonator circuits may be used in shunt with the high power amplifieroutput and may be tuned so that the phase delay at any harmonicfrequency that needs to be suppressed uses the harmonic signal to createcancelation. Multiple coupled resonators may be used for shorting outadditional harmonic frequencies. The coupled resonator circuits may alsobe actively tuned during a LFM waveform so the harmonic traps can movedynamically with the fundamental and lower the unwanted harmoniccontent.

As illustrated in FIG. 2, harmonic trap filter 130 may comprise an inputterminal 202 and a ground terminal 204 electrically coupled to aplurality of resonators 210 a-210 f, representatively referred to hereinas resonator(s) 210. Each resonator 210 may comprise an inductor L1-L6,respectively, and a variable capacitance C1-C6, respectively.

Turning momentarily to FIG. 3, them is illustrated an example variablecapacitance circuit 300 that may be used to embody the principles ofthis disclosure. Variable capacitance circuit 300 may be realized atC1-C6 illustrated in FIG. 2. Variable capacitance may be achieved byapplying a variable DC voltage to varactor VR310 by a voltage sourceV(A), where A is an amplitude of the DC capacitance control signal. Incertain embodiments, V(A) can be varied with sufficient rapidity todynamically modify the capacitance, and hence the resonant frequency ofthe corresponding resonator 210, during non-stationary frequencywaveforms, e.g., LFM waveforms.

Each variable capacitance circuit 300 may further be coupled to an ACvoltage source V(θ), where θ is a phase angle relative to that of otherresonators 210. The purpose of this phase angle is described in moredetail below. In certain embodiments, the amplitude and frequency ofV(θ) is static across all resonators 210, with the frequency being muchlower than that of harmonic frequencies being filtered. For example, fora 1 GHz resonator, the modulation frequency may be 70-210 MHz. In otherembodiments, the amplitude of AC voltage source V(A) may be varied toalter the capacitance of varactor VR 310, in which case DC voltagesource V(A) may be held constant.

It is to be understood that variable capacitance circuit 300 may beimplemented in ways other than that illustrated in FIG. 3. Those havingskill in microwave circuits will recognize numerous variable capacitancetechniques that can be used without departing from the spirit of theprinciples described herein. The filtering and choke elements, C302,L304, L306, L308, C312, L314 and L316 may be chosen according theoperating frequencies of the application being implemented. Moreover, itis to be understood that techniques other than variable capacitance canbe used to realize the principles described herein, which will beapparent to skilled artisans upon review of this disclosure.

Returning to FIG. 2, it is to be observed that resonators 210 arecoupled one to another in an order defined by the phase shift θ of theAC voltage applied thereto. For example, resonator 210 a may have an ACvoltage V1(0) applied thereto, resonator 210 b may have an AC voltageV2(120) applied thereto and resonator 210 c may have an AC voltageV3(240) applied thereto. Similarly, resonator 210 d may have an ACvoltage V4(120) applied thereto, resonator 210 e may have an AC voltageV5(240) applied thereto and resonator 210 f may have an AC voltage V6(0)applied thereto. The phase angle differences are applied in, forexample, 120° increments around sets of resonators 210 to definespatiotemporally modulated loops of resonators 210. It is to beunderstood, however, that other phase increments, e.g., 90°, may be usedto embody the principles described herein. Additionally, it is to beunderstood that while resonators 210 are illustrated as beingwye-connected, other topologies, such as delta-connected resonators mayalso implement the principles of this disclosure. These spatiotemporallymodulated loops of resonators 210 realize non-reciprocity with respectto ports P1-P3 and P4-P6. When so embodied, resonators 210 implement apair of coupled circulators 220 a and 220 b, representatively referredto herein as circulator(s) 220. Bandpass filters F1-F6 contain the biasvoltages V1-V12 within circulators 220.

FIG. 4 is an electrical schematic diagram of harmonic trap filter 130 ata higher level of abstraction than that illustrated in FIG. 2. It is tobe understood that circulators 220 a and 220 b can be implemented byspatiotemporal modulation and not by magnetic bias by permanent magnets.As used herein, such circulators are referred to herein as “magnet-free”circulators. By way of the principles described herein, resonators ofcirculators 220 a and 220 b can be tuned to impart a 90° phase delayfrom port to port. That is, at resonance (where the losses from P1 to P2in circulator 220 a and from P4 to P5 in circulator 220 are minimum andthe isolation between P1 to P3 in circulator 220 a and between P4 to P6in circulator 220 b are maximum) there may be a 90° phase delay fromport P1 to port P2 of circulator 220 a and an additional 90° phase delayfrom port P4 to port P5 of circulator 220 b for a total of 180° phasedelay through both circulators 220 a and 220 b. Thus, at the chosenharmonic frequency for which harmonic trap filter 130 is tuned, there isa cancelation of the harmonic signal at the input node 202.

As illustrated in FIG. 4, port P3 of circulator 220 a and port P6 ofcirculator 220 b are terminated in respective resistive loads, RX and RY(also shown in FIG. 2). As such, circulators 220 a and 220 b formrespective isolators that define a non-reciprocal electrical path thatbegins and ends at input terminal 202, i.e., from input terminal 202,through ports P1 and P2 of circulator 220 a, through ports P4 and P5 ofcirculator 220 b and terminating at input terminal 202. As stated above,a harmonic signal component that traverses this circuit path can undergoa 180° phase delay to effect cancelation of the harmonic signalcomponent at input terminal 202. Resistors RS and RL may be used forsource and load impedance matching, respectively.

FIG. 5 is a graph of simulated transmission coefficient S(2,1) of theembodiment illustrated in FIG. 4. At the operating frequency of 1.0 GHz,them is little to no insertion loss, due to the shunt configuration ofthe harmonic trap filter 130. At the second harmonic of 2.0 GHz,however, there is a reduction of almost 10 dB through cancelation.

FIG. 6 is an electrical schematic diagram of another harmonic trapfilter 630 by which the principles described herein may be embodied.Harmonic trap filter 630 may be constructed or otherwise configured toremove multiple harmonics from a signal applied to its input node 602.To that end, harmonic trap filter 630 may include a first isolator pair610 a comprising isolators 620 a and 620 b, and a second isolator pair610 b comprising isolators 620 c and 620 d. Isolator pairs 610 a and 610b are representatively referred to herein as isolator pair(s) 610 andisolators 620 a-620 d are representatively referred to herein asisolator(s) 620 (or isolator circuit(s) 620). Isolators 620 may beconstructed from circulators, each terminated at respective third portsby an appropriate load. As illustrated in FIG. 6, isolator 620 a isconstructed from a circulator terminated in RX at its port P3, isolator620 b is constructed from a circulator terminated in RY at its port P6,isolator 620 c is constructed from a circulator terminated in RV at itsport P9 and isolator 620 d is constructed from a circulator terminatedin RW at its port P12. Resistors RS and RL serve as example source andload impedance matching circuits, respectively.

Each isolator pair 610 may be constructed and may operate in the mannerillustrated in FIGS. 2 and 3, although, as mentioned above, othercircuit topologies may be employed to realize the spatiotemporalmodulated loops of resonators. Isolator pairs 610 define respectivenon-reciprocal electrical paths that begin and end at input terminal602. That is, a first electrical path proceeds from input terminal 602,through ports P1 and P2 of isolator 620 a, through ports P4 and P5 ofisolator 620 b and terminates at input terminal 602. A second electricalpath proceeds from input terminal 602, through ports P7 and P8 ofisolator 620 c, through ports P10 and P11 of isolator 620 d andterminates at input terminal 602. Each isolator pair 610 may be tuned todifferent harmonic signal components of the signal applied to inputterminal 602 such that, at each harmonic frequency, the correspondingharmonic signal component undergoes a 180° phase delay or shift when thecorresponding electrical path is traversed thereby. The 180° phase delayapplied to each harmonic effects cancelation of that harmonic at inputnode 602.

FIG. 7 is a graph of simulated transmission coefficient S(2,1) of theembodiment illustrated in FIG. 6. At the operating frequency of 1.0 GHz,there is little to no insertion loss, due to the shunt configuration ofthe harmonic trap filter. At the second harmonic of 2.0 GHz and at thethird harmonic of 3.0 GHz, however, there is a reduction of almost 10 dBthrough cancelation. Referring back to FIG. 6, isolator pair 610 a istuned to cancel the second harmonic of 2.0 GHz and isolator pair 610 bis tuned to cancel the third harmonic of 3.0 GHz.

It is to be understood that the concept of canceling more than oneharmonic signal component described with reference to FIG. 6 may beextended to any number of harmonics by incorporating additional isolatorpairs as needed.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the principlesdescribed herein. As used herein, the singular forms “a”, “an” and “the”are intended to include the plural forms as well, unless the contextclearly indicates otherwise. It will be further understood that theterms “comprises” and/or “comprising,” when used in this specification,specify the presence of stated features, integers, steps, operations,elements, and/or components, but do not preclude the presence oraddition of one or more features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription within this disclosure has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the forms disclosed. Many modifications and variations willbe apparent to those of ordinary skill in the art without departing fromthe scope and spirit of the principles described herein.

The descriptions above are intended to illustrate possibleimplementations of the present concept and are not restrictive. Manyvariations, modifications and alternatives will become apparent to theskilled artisan upon review of this disclosure. For example, componentsequivalent to those shown and described may be substituted therefore,elements and methods individually described may be combined, andelements described as discrete may be distributed across manycomponents. The scope of the principles should therefore be determinednot with reference to the description above, but with reference to theappended claims, along with their full range of equivalents.

1. A harmonic trap filter to suppress at least one harmonic signalproduced by an amplifier, the harmonic trap filter comprising: an inputterminal and a ground terminal; and a plurality of resonatorselectrically coupled one to another between the input terminal and theground terminal in a spatial order defined by relative phase shift ofalternating voltage bias signals respectively applied thereto, theresonators being tuned to resonate at at least one frequency at which aphase delay is imparted to the at least one harmonic signal by theresonators to effect cancelation of the at least one harmonic signal atthe input terminal.
 2. The harmonic trap filter of claim 1, wherein theresonators are electrically coupled into isolator circuits connected inseries to define at least one non-reciprocal electrical path that beginsand ends at the input terminal.
 3. The harmonic trap filter of claim 2,wherein the series-connected isolator circuits are series-connectedcirculators.
 4. The harmonic trap filter of claim 3, wherein a firstcirculator of the series-connected circulators is electrically coupledin series with a second circulator of the series-connected circulators,where an input port of the first circulator and an output port of thesecond circulator are electrically coupled to the input terminal.
 5. Theharmonic trap filter of claim 2, wherein the relative phase shiftapplied to the resonators of each isolator circuit is a predeterminedphase angle apart.
 6. The harmonic trap filter of claim 5, wherein thepredetermined phase angle is 120°.
 7. The harmonic trap filter of claim2, wherein the resonators electrically coupled into isolator circuitsconnected in series define respective non-reciprocal electrical pathsthat begin and end at the input terminal, the isolator circuits in eachnon-reciprocal electrical path being tuned to resonate at a frequencythat is different from a frequency at which the isolator circuits inother non-reciprocal electrical paths resonate.
 8. The harmonic trapfilter of claim 1, further comprising: control circuitry to generatecontrol signals, wherein the resonators each comprise a variablecapacitive element by which resonance of the corresponding resonator istuned, the capacitance of each variable capacitive element beingestablished by a corresponding one of the control signals appliedthereto by the control circuitry.
 9. The harmonic trap filter of claim8, wherein the control circuitry is configured to generate the controlsignals to vary over time such that the frequency at which the phasedelay is imparted to the at least one harmonic signal varies inaccordance with a frequency modulated signal applied to the inputterminal.
 10. A radio-frequency (RF) transmitter comprising: anamplifier; and a harmonic trap filter to suppress at least one harmonicsignal produced by the amplifier, the harmonic trap filter comprising:an input terminal and a ground terminal; and a plurality of resonatorselectrically coupled one to another between the input terminal and theground terminal in a spatial order defined by relative phase shift ofalternating voltage bias signals respectively applied thereto, theresonators being tuned to resonate at at least one frequency at which aphase delay is imparted to the at least one harmonic signal by theresonators to effect cancelation of the at least one harmonic signal atthe input terminal.
 11. The RF transmitter of claim 10, wherein theresonators are electrically coupled into isolator circuits connected inseries to define at least one non-reciprocal electrical path that beginsand ends at the input terminal.
 12. The RF transmitter of claim 11,wherein the series-connected isolator circuits are series-connectedcirculators.
 13. The RF transmitter of claim 12, wherein a firstcirculator of the series-connected circulators is electrically coupledin series with a second circulator of the series-connected circulators,where an input port of the first circulator and an output port of thesecond circulator are electrically coupled to the input terminal. 14.The RF transmitter of claim 11, wherein the relative phase shift appliedto the resonators of each isolator circuit is a predetermined phaseangle apart.
 15. The RF transmitter of claim 14, wherein thepredetermined phase angle is 120°.
 16. The harmonic trap filter of claim11, wherein the resonators electrically coupled into isolator circuitsconnected in series define respective non-reciprocal electrical pathsthat begin and end at the input terminal, the isolator circuits in eachnon-reciprocal electrical path being tuned to resonate at a frequencythat is different from a frequency at which the isolator circuits inother non-reciprocal electrical paths resonate.
 17. The RF transmitterof claim 10, further comprising: control circuitry to generate controlsignals, wherein the resonators each comprise a variable capacitiveelement by which resonance of the corresponding resonator is tuned, thecapacitance of each variable capacitive element being established by acorresponding one of the control signals applied thereto by the controlcircuitry.
 18. The RF transmitter of claim 17, wherein the controlcircuitry is configured to generate the control signals to vary overtime such that the frequency at which the phase delay is imparted to theat least one harmonic signal varies in accordance with a frequencymodulated signal applied to the input terminal.
 19. A shunt harmonictrap filter comprising: an input terminal; and a first magnet-freeisolator circuit and a second magnet-free isolator circuit electricallycoupled in series with the first magnet-free isolator circuit to definea non-reciprocal circuit path that begins and ends at the inputterminal.
 20. The shunt harmonic trap filter of claim 19, wherein eachof the first magnet-free isolator circuit and the second magnet-freeisolator circuit comprise a plurality of resonators, each being tuned toresonate at a frequency at which a phase delay is imparted to a harmonicsignal by the first and second magnet-free isolator circuits to effectcancelation of the harmonic signal at the input terminal.